1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine of an automobile and the like.
2. Description of Related Art
A spark plug is used for igniting a fuel-air mixture introduced into a combustion chamber of an internal combustion engine of an automobile and the like. The spark plug described in Japanese Patent Application Laid-open No. H1-279588 includes a center electrode, an insulator holding the center electrode inserted therein, a housing holding the insulator inserted therein, and a ground electrode joined to the housing so as to form a spark discharge gap with the center electrode. A stem electrically connected to the center electrode is inserted into the insulator from the proximal side of the insulator opposite to the side of the center electrode (the distal side). To electrically connect the spark plug to an ignition coil, the proximal end portion of the insulator (the insulator head portion) is fitted to a rubber plug cap.
However, the above described spark plug has a problem as described below. There has been a growing trend to employ supercharging or increase compression ratio of an internal combustion engine for the purpose of increasing fuel economy of the engine. As a result, the temperatures of combustion chambers tend to increase. When the temperature of a combustion chamber to which the distal end portion of the spark plug is exposed increases, since an amount of heat transferring from the distal end portion of the center electrode to the stem increases, the temperature of the insulator head portion easily increases. In this case, the rubber plug cap to which the insulator head portion is fitted undergoes heat deterioration easily.